HIV-1 Vpr is a nuclear, virion packaged accessory protein encoded by all lentiviruses. SIV/HIV-2 encodes in addition, Vpx which maintains close sequence similarity. The hallmark of Vpr is that it induces G2/M cell cycle arrest. A key to understanding the role of Vpr/Vpx in virus replication and pathogenesis is the identification of the cellular proteins with which they interact. Several groups, including ours, recently reported the identification of a cellular complex with which Vpr and perhaps Vpx interact and through which they may function. The complex is an E3 ubiquitin ligase that consists of DDB1/DCAF1/CUL4A/ROC1. This complex regulates genomic DNA replication and damaged DNA repair by mediating the destruction of cellular proteins including the DNA replication-licensing factor CDT1, STAT1 and STAT2 and other, yet to be identified substrates. The identification of the interaction with the E3 ubiquitin ligase presents a unified explanation of how Vpr may mediate its effects and provides a framework to elucidate the role that Vpr plays in HIV-1 pathogenesis. Other viruses encode proteins that target this complex, using it as a means to induce degradation of STAT proteins to inhibit type-I interferon responses. We will determine whether Vpr acts through its interaction with the complex to arrest cells in G2, induce apoptosis and activate DNA damage signaling pathways. In addition, cellular proteins that bind to Vpx and to DCAF1 will be identified and characterized. Thirdly, the project will test they hypothesis that Vpr/Vpx play a role in dampening innate immune responses to the virus. Innate immunity has over the past several years become a topic of great interest. The project will determine whether Vpr/Vpx may constitute another such mechanism. PUBLIC HEALTH RELEVANCE Cells have mechanism by which they resist infection by viruses. These mechanism are of great interest because they provide new avenues for the development of antiviral drugs. Viruses have evolved means of escaping these so called innate immune mechanisms. HIV, for example produces a protein called virion infectivity factor( VIF) that inactivates one of the antiviral proteins that are produced by blood cells. In this project we will study an HIV protein called viral protein R (VPR) that may serve as yet another means by which the virus avoids the innate immune mechanisms. When a cell is infected with HIV, Vpr is produced and as we recently discovered, binds to a cellular protein called damaged DNA binding protein 1 (DDB1) in association with another protein called DCAF1. DCAF1/DDB1 is a molecular machine of the cell that destroys unwanted proteins. Our findings have shown that Vpr causes DDB1 to degrade certain cellular proteins and we hypothesize that at least one of these is an antiviral protein that the virus needs to destroy in order to replicate. The project will identify the targets of DDB1 and understand how these proteins work against the virus. In addition, Vpr seems to affect the normal function of the DCAF1/DDB1 complex and thereby disrupt functions in the cell such as DNA repair and DNA replication. The project will determine how this happens and how it affects replication of HIV.